This invention relates generally to metallic alloys, and more particularly to a new class of amorphous metallic alloys.
Until now it has not been believed possible to obtain an alloy where one metal is immiscible in the other metal. Such an alloy, if it could be obtained, would present several advantages. Totally new alloy systems would become available. Since one of the metals of the alloy is immiscible in the other, it may be assumed that there is minimal interaction between the atoms of the immiscible metals. Hence, one could dilute a metal with a second metal which is immiscible in the first metal without significantly altering the atomic properties of either metal. For example, bismuth could be added to iron or cobalt to improve the switching characteristics of the iron or cobalt, without significantly reducing the atomic magnetic moments of iron or cobalt.
Currently, there are no known intermetallic compounds, eutectic alloys, or solid solutions of any type incorporating iron or cobalt and bismuth. Bismuth is almost totally insoluble in iron or cobalt and its melting point (544 K.) is less than one third that of iron (1768 K.).
U.S. Pat No. 4,014,688 to Schreiner et al. discloses an alloy composed of a eutectic mixture of iron crystals in boron, the mixture having a small amount (about 0.3 weight percent) of precipitated bismuth being finely dispensed at the boundaries of the iron crystals. In U.S. Pat. No. 3,762,915 to Heine et al., an extremely small amount of bismuth (about 0.0005 to 0.02 weight percent) is added to a gray casting iron composition in order to alter the freezing characteristics of the composition. Neither of these alloys, however, are true alloys of bismuth. Instead, extremely small amounts of bismuth exist as precipitates throughout the alloy. The relatively low freezing point of bismuth as opposed to that of iron exaggerates the tendency of bismuth to segregate from iron or steel while the mixture solidifies. Of course, a similar difficulty occurs when one attempts to alloy cobalt or nickel with bismuth and when one attempts to alloy immiscible metals in general. In contrast to these prior iron or steel-bismuth compositions, novel Fe-Bi alloys of this invention are about fifty percent bismuth by volume (about 40-50 weight-percent bismuth). In this description, the notations (Fe,Co)-Bi, (Fe,Co)-bismuth and (Fe,Co).sub.1-x Bi.sub.x refer to alloys of iron and bismuth and alloys of cobalt and bismuth. Notations such as Fe-Bi and Mn-Bi are merely shorthand for the more formal notations employing subscripts.
The desirability of an amorphous iron, cobalt, or nickel-bismuth alloy derives from the alloys' unique ferromagnetic and magneto-optical properties. Often, data is stored in memory by magnetically recording data on reflective ferromagnetic film. The film is read by directing a polarized laser beam onto the surface of the film. The magnetic domains in the region where the light strikes rotate the plane of polarization of the light reflected from the film and causes it to differ from that of the incident beam. This change or rotation of the plane of polarization is referred to as the Kerr rotation. The extent of the Kerr rotation observed depends on the composition of the ferromagnetic film and the wavelength of the incident light. By selecting the appropriate combination of ferromagnetic material for the film and wavelength of incident light, one can determine the configuration of the magnetic domains in the film and recover the information recorded thereon. Of course, one can also recover information from the film by conventional playback means as well.
From the standpoint of practical applications, Mn-Bi crystalline alloys are known to have the highest magneto-optical Kerr rotations of any alloy system to date and are frequently used in magneto-optical mirrors and magnetic-optic memory applications. Since the Kerr rotation in general is larger for larger magnetization alloys of the transition elements, it could be expected that in some wavelength regions, Fe-Bi and Co-Bi should outperform crystalline Mn-Bi because iron and cobalt have much larger magnetic moments than manganese.
For magnetic storage of data, it is desirable that the switching characteristics of the film should be as square as possible. That is, the film should have as few pinning defects as possible so that the walls of the magnetic domains within the film can expand freely in response to an external magnetic field. Of course, the magnetic domains must be capable of aligning in the magnetic field used for magnetic recording and yet not be so responsive that stray magnetic fields cause them to lose their alignment.
Further, it is desirable that a recording film be able to store as much data as possible in as small a space as possible. When, as is typical, the magnetic domains of the magnetic recording film are oriented parallel to the film surface, each domain exerts some influence upon its neighboring domains. Therefore, when recording upon magnetic film, care must be taken so that the magnetic domains along the film are not overly crowded. Thus, the amount of data that can be stored within any one area of film becomes severely limited. On the other hand, if the magnetic domains are oriented perpendicular to the film surface, the problem of domains interacting with neighboring domains is greatly allieviated. Therefore, more data can be stored on a film which has its domains oriented perpendicular to the film surface than on a film which has its domains oriented parallel to the film surface. Previously, a film having a large net magnetization and its domains oriented perpendicular to the film surface was quite difficult to obtain.
While elemental iron and cobalt are strongly ferromagnetic and exhibit large Kerr rotations, films of elemental iron or cobalt have relatively poor switching and memory storage characteristics. Alloys, frequently crystalline, of iron or cobalt with other elements have been used to attempt to overcome these difficulties. Most recently, films of transition metal-metalloid metallic glasses, such as Fe-B, have been used for magneto-optical applications. These glasses, however, generally have magnetic domains oriented in the film plane, reduced magnetization of the transition metal atoms, and strong tendencies toward oxidation. Therefore, the handling of these glasses is a difficult and delicate matter.
Although, the alloys of this invention are amorphous, they differ significantly from metallic glasses, such as Fe.sub.1-x B.sub.x. While the novel alloys are, like metallic glasses, most stable near the x=0.2 region, unlike metallic glasses, the novel alloys have no crystalline phase diagrams. Further, the novel alloys of this invention have a majority of metallic atoms with radii that are significantly smaller than the radii of the minority of the atoms in the metastable alloy, in contrast to the metallic glasses.
Although amorphous alloys of Mn.sub.0.8 Bi.sub.0.2 have been made using a process similar to that used to produce the novel alloys of this invention, the constituent elements of Mn.sub.0.8 Bi.sub.0.2, manganese and bismuth, are miscible, unlike the constituent metals of alloys of this invention. Therefore, the existence of an amorphous alloy of manganese and bismuth would not cause one to suspect that an amorphous alloy of two immiscible metals could be obtained. Further, amorphous Mn.sub.0.8 Bi.sub.0.2 has no magnetic moment down to 78 K., while several of the alloys of this invention, for example Fe.sub.0.86 Bi.sub.0.14, are usefully ferromagnetic even at room temperature and above.